Power brake



D e m8, 1942. H. BAADE ET AL 2,304,621

' POWER BRAKE I Fuea Aug. 21. 1940 4 Sheets-Sheet 1" HENRY BAADE BY THOMAS H. THOM AADE AL v P 'Q POWER BRAKE Filed Aug. 21, 194i) 4; Sheets-Sheet 2 INVENTO HENV EQ E THQMA H. THOR/3A5 I Dec; 8, 1942. H. AADE ETAL.

POWER BR KE Filed Aug. 21, 1940 4 Sheets-Sheet 4 mvmvrom ENQV @QDE THDfifi fiHOMQ$ Patented Dec. 8, 1942 POWER BRAKE Henry Baade, Detroit, Micln, and Thomas H. Thomas, South Bend, Ind., assignors to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application August 21, 1840, Serial No. 353,458

Claims.

This invention relates to fluid pressure brake systems, and more particularly to an improved control valve for fluid pressure brakes.

It is often desired to control one set of brakes in response to another. This is particularly true in a tractor and trailer combination where it is necessary to have adequate brakes for both vehicles and at the same time provide for control by the operator over the brakes of both vehicles.

In tractor and trailer braking combinations it is usually advantageous to provide a fluid pressure power motor for the tractor brakes and a second fluid pressure power motor for the trailer brakes. The operator then usually controls the tractor power motor which, in turn, controls the operation of the trailer power motor.

A power cylinder or motor designed to use vacuum power may be either one of two typesvacuum suspended in release or atmospheric suspended in release. It is therefore evident that a tractor-trailer power braking system may have any one of four power cylinder combinationsvacuum suspended on the tractor with vacuum suspended on the trailer, atmospheric suspended on the tractor with atmospheric suspended on the trailer, vacuum suspended on the tractor with atmospheric suspended on the trailer, and atmospheric suspended on the tractor with vacu suspended on the trailer.

In any tractor-trailer power motor combination it is desirable and even necessary to provide a relay valve, located on the trailer near the trailer cylinder to be actuated by the pressure in the control line coming from the tractor and to itself actuate the trailer power motor. The relay, or speeding up function of the valve, is needed in all four of the above-mentioned combinations.

The vacuum tractor-atmospheric trailer and atmospheric tractor-vacuum trailer combinations both require an additional function from the relay or control valve. This additional function is the conversion of pressure impulses from the tractor cylinder into pressure which will actuate the trailer cylinder. The direction-of the pressure change which occurs during brake application is thuschanged between the tractor and trailer power motors.

The control valve which we have invented capable of use with any of the four combinations, and has when used the relay function and the conversion function where necessary. Because of its adaptability we choose to call our valve a universal relay valve.

An important object of our invention is the provision of a universal relay valve which will operate to seal effectively and to accurately proportion the eflective pressures of the associated D wer motors.

Our valve has the further advantage that, when combined with the proper power brake hookup,

" others.

it will provide the automatic trailer braking upon breakaway required by the laws of several states.

Another advantage of our invention is that the valve elements which control the operation of the trailer power brakes are of the poppet type and are adapted to seal effectively when the of the tractor is vacuum suspended and that of the trailer vacuum suspended;

Figure 3 shows an atmospheric to atmospheric tractor-trailer power brake hookup;

Figure 4 shows a vacuum to atmospheric hookup;

Figure 5 shows an atmospheric to vacuum hookup; and

Figure 6shows a modification of the valve 01' Figure l.

Proceeding now to a detailed discussion of the preferred embodiment, as shown, we flnd in Figure 1 a valve Iii having a lower casing H and an upper casing ii, the two parts'being secured together by a plurality of bolts or other suitable securing means. A cover or cap element I3 is fastened to thehpper casing I! by any suitable means. A diaphragm i4 is secured between the elements ii and I2 and a second diaphragm I5 is secured between the elements l2 and I3. An annular partition l6 integral with the element l2 divides into two parts the interior of the cylinder lying between diaphragms i4 and I5. The casing elements, diaphragms, and the partition thus divide the valve i0 into four chambers, H, i8, i9, and 28, each of which is sealed from the A rod 2i is clamped to diaphragm I6 and extends downward through the partition l8 and the diaphragm It. When the parts of the valve are operated the two diaphragms and the valve move together, the center of the rod moving upward or downward through the partition i6. Annular sealing elements 22 and 23 surround rod 2| where it passes through partition l6, effecting the sealing of chamber it from chamber IS. The element 22 also serves as a flexibly mounted bushing for the rod II. The diaphragms I4 and II are each provided with washers 24 clamped against the diaphragms adjacent the rod 2 I, for further sealing the several chambers from each other.

Ports 25, 20, and 21 open respectively into the chambers I1, I8, and I9. Chamber 20 is provided with ports 28, 29, and 30. In each of the various power brake hookups hereinafter described, port 28 is open to atmosphere, preferably through an air cleaner, port 29 is connected to a source of vacuum, and port 30 is connected to the trailer power motor.

The ports 28 and 29 are adapted to be closed at times by poppet type valve elements 32 and 33. The movements of the valve elements 32 and 33 are controlled by their stems, 34 and 35 respectively, which in turn are attached by pins 38 and 31 to a triangular lever 38. A third pin 99 fixes one corner of the lever 38 to the lower end of the rod 2|. Movement of the rod 2| will therefore move the lever 38 to alternately open and close the ports 28 and 29. It will be seen that downward movement of rod 2| will first actuate valve element 33 to close port 29 and then actuate valve element 32 to open port 28. Upward movement of rod 2| from its lowermost position will first close port 28 and then open port 29. It is to be noted that there is a position of rod 2| in which both ports 28 and 29 will be closed, this position being the lapped position of the valve.

The movements of the rod 2| are subject to control by two forces, the force applied by the pressure differential over diaphragm I4 and the force applied by. the pressure difierential over diaphragm I5. The relation of these two forces will determine the position and movement of rod 2|.

The use of our improved valve in each of the several power brake hookups will now be described. Figure 2 illustrates its use in a vacuum suspended tractor-vacuum suspended trailer hookup. A tractor I and a trailer I0| are provided respectively with wheels I02 and I03 having associated therewith brakes indicated generally at I04'and I05. The tractor brakes I04 are adapted to be applied through suitable brake linkage by force supplied from a vacuum suspended power cylinder IIIG. A vacuum suspended power cylinder I01 is provided for actuating the trailer brakes I05. The front end of the tractor power cylinder I06 is connected by a conduit I08 to the manifold I09 of the tractor's internal combustion engine. in the conduit I08. A conduit III, branching from conduit I08 connects the manifold I09 to a control valve H2. The valve H2 is controlled by the operator of the tractor to admit alternately atmosphere and vacuum to one side of the tractor power cylinder. A conduit 3 connects the control valve |I2 to the rear end of power cylinder I06. A conduit 4 connects conduit II3 through the port 25 to chamber I1 of the valve I0, which is shown mounted on the trailer. A conduit II5 connects the conduit I08 to a vacuum reserve tank II6 on the trailer. A branch conduit 1 connects-the conduit 5 to the front end of power cylinder I01. A check valve H8 is preferably placed in conduit II5 between the intersection of conduit 5 with conduit I08 and its intersection with conduit II1. Connecting port 29 of chamber '20 of the valve I0 to the conduit 1 is a branchconduit II9. Chambers I8 and I9 of the valve .I0 are connected by their respective lines "I20 and |2I to an air cleaner I22, open at all times to air at atmospheric pressure. A line I23 connects port 30 of the valve I0 to the rear end of power cylinder I01. Port 28' of valve I0 is connected to the air cleaner I22.

A check valve H0 is interposed The operation of the power brake hookup shown 'in Figure 2 will now be described. In released position of the brakes the pistons in power cylinders I08 and I01 are submerged in vacuum, the control valve I0 admits vacuum from the conduit 9 to conduit I23 and the rear end of the tractor power cylinder, and the valve I0 is substantially as shown in Figure 1, with port 28 closed and port 29 open. When the operator wishes-to apply the brakes, he pushes a brake pedal (not shown) to crack the valve II2. This results in closing of! the vacuum and connecting atmosphere to conduit H3. The atmosphere in the conduit 3 enters the rear end of cylinder I08, creating a pressure differential over the piston of power cylinder I05 and thus moving said piston to apply the tractor brakes.

Part of the air entering into conduit H3 is transmitted to chamber I1 of the universal relay 'valve I0. Prior to the entry of air into chamber I1, the valve I0 has been in released position. This is true because chambers I1 and 20 have been connected to vacuum, while chambers I8 and I9 have been connected to atmosphere. Since the area of the diaphragms I4 and I5 is approximately equal, the pressure of the air in chamber I8 tending to push diaphragm I5 upward has been balanced by the air pressure in chamber I9 tending to push diaphragm I4 downward. A small spring I24 has helped to hold the valve in released position.

When air enters chamber I1 it unbalances the pressure on rod 2| and the rod begins to move downward. This causes movement of the triangular lever 38 to first close port 29 cutting off the vacuum and then open port 28 admitting air to chamber 20, and thence through port 30 and conduit I23 to the rear end of the trailer power cylinder Ifl'Lmoving the piston therein to apply the trailer brakes.

It will be noted that the admission of air into chamber 20 will cause an increase of the upward pressure on diaphragm I4, which will eventually equal the downward pressure in chamber I1 and cause rod 2| to discontinue its downward movement. Further actuation of the trailer brakes will then require further actuation of the tractor brakes. Thus the trailer brakes are operated in accordance with the amount of pressure applied to the tractor brakes, and the braking of the two vehicles is proportional.

In case of breakaway, the check valve II8 will prevent diminution of the trailer vacuum reserve. At the same time the breaking of conduit II4 will allow air to enter chamber I1 of the valve I0 and apply the trailer brakes.

Figure 3 shows an atmospheric suspended tractor-atmospheric suspended trailer hookup. In this instance, air suspended power cylinders I25 and I26 are provided to apply the brakes of the tractor and trailer respectively. The cylinder I25 of the tractor is connected by a conduit I21 to a control valve I28, arranged to admit air to conduit I21 when the brakes are released, but to close ofi the air whenever the brakes are applied and connect conduit I21 to conduit I29 which leads to the manifold of the tractor engine, the preferred source of vacuum. A check valve I30 is placed in the'conduit I29. A branchconduit I3I leads from conduit I21 to the chamber I9 of valve I0 on the trailer. ing a check valve I33, extends from conduit I29 to a vacuum reserve tank I34 on the trailer. Conduit I32 has a branch line I35 leading there- .from to chamber I8 of the valve I0, and a second A conduit I32, havbranch line I36 leading to port 29 of the valve II. The line I35 intersects conduit I32 on the manifold side of the check valve I33, whereas the line I36 intersects conduit I32 on the reserve tank side of the check valve I33. A conduit I31 connects chamber I1 01' the valve I8 to the branch line I36. Port 38 of valve I8 is connected by conduit I38 to the front end of the trailer power cylinder, and port 28 of valve I8 is open to atmosphere through the air cleaner I22.

It will be apparent, therefore, that, in this hookup, the chambers of the valve I8 are connected as follows: chamber I1 to the reserve vacuum tank, chamber I8 to the vacuum on the manifold side of the check valve, chamber I9 to the control line, and chamber 28 to air, vacuum and the trailer power cylinder. When the brakes are in released position, the pistons of power cylinders I25 and I26 are suspended in atmosphere, while rod 2I in valve I8 is at its lowermost position, admitting air through ports 28 and 38 to the trailer power cylinder. Rod 2| is held at its lowermost position during brake release because the diaphragm I is suspended in vacuum while the air in chamber I9 exerts a downward pressure on diaphragm I4. The downward force will be balanced when suflicient air has been admitted to chamber 28 through port 28 to counteract the air pressure in chamber I9. Actuation of the tractor brakes admits vacuum to conduit I3I and thence to chamber I9 of the valve I8. Since chamber 28 is open to air at atmospheric pressure during the time the trailer brakes remain released, the admission of vacuum to chamber I9 will unbalance the pressures over diaphragm I4 and move the rod 2| upward, closing the air port 28 and opening the vacuum port 29. Vacuum is thus admitted through ports 29 and 38 to the forward end of power cylinder I26, causing the application of the trailer brakes.

In this hookup, as in that of Figure 2, the braking of the tractor and trailer vehicles will be proportional, since the same pressure which enters the trailer power cylinder to apply the trailer brakes also exerts a force on the control diaphragm I4 which results eventually in .bringing the valve I8 to lapped position and preventing further application of the trailer brakes. In case of breakaway of the trailer from the tractor, check valve I33 prevents instant depletion of the vacuum reserve in the tank I34. At the same time breaking of the conduit I32 admits air through conduit I35 to chamber I8 of the valve I8, unbalancing the pressures over diaphragm I5 and moving the rod 2I upward to cause application of the trailer brakes.

Figure 4 shows a hookup in which a vacuum suspended power cylinder on a tractor is combined with an atmospheric suspended power cylinder on a trailer. The forward end of a vacuum suspended power cylinder I 58 on the tractor vehicle is connected by a conduit I5I to the manifold of the tractor engine. Interposed in the conduit I5I is a check valve I52 and connecting the said conduit to a pedal actuated valve I53 is a conduit I54. The valve I53 is connected by a conduit I55 to the rear of the cylinder I58. A branch conduit I56 connects the conduit I5I to a vacuum reserve tank I51 on the trailer on the trailer vehicle, a check valve I58 being interposed in the conduit I56. A branch conduit I59 connects the conduit I55 through port 26 to chamber I8 of the valve I8 on the trailer. An air cleaner I68 on the trailer is connected by conduits I6I and I62 respectively to chamber I9 of the control valve I8 and to port 28 of the valve I8. A-branch conduit I63 connects vacuum conduit I56 to port 29 of the valve I8 and a branch conduit I64 connects conduit I63 to port 25. An atmospheric suspended power cylinder I 65 on the trailer is connected at its forward end to a conduit I66 which in turn is connected to port 38 of the valve I8.

When used in the hookup of Figure 4, our universal relay valve has the following connections: chamber I1 is connected to the reserve vacuum tank, chamber I8 is connected to the control line, chamber I9 is connected to atmosphere. As long as the trailer brakes remain released, they are suspended in atmosphere and air at atmospheric pressure is present in chamber 28, the rod 2| resting during brake release in its lowermost position. The diaphragm I4 is therefore balanced between two atmospheric chambers and the diaphragm I5 is balanced be? tween two vacuum chambers, since the control line admits vacuum to the chamber I8 'so long as the vacuum suspended brakes of the tractor are released. When the pedal on the tractor is actuated to crack the valve I53, air is admitted through the conduit I55 to the rear of the cylinder I58 moving the piston therein to apply the tractor brakes. At the same time air moves through conduits I 55 and I59 to chamber I8 of the valve I8, unbalancing the pressures therein and moving the rod 2| upward against the vacuum in chamber I1. The air port 28 is closed by this movement and the vacuum port 29 is open, admitting vacuum to the forward end of the trailer power cylinder I65, and causing application of the trailer brakes. A breakaway of the trailer vehicle from the tractor would break the conduit I 59, resulting in the admission of air through the said conduit to the chamber I8 to cause actuation of the trailer brakes.

Figure 5 shows a tractor-trailer hookup in which an atmospheric suspended power cylinder I15 on the tractor and a vacuum suspended power cylinder pnthe trailer provide the braking power. A conduit" I11 connects the manifold of the tractor to the pedal controlled valve I18. The valve I18 is connected by a conduit I19 to the forward end of the power cylinder I15, and a branch conduit I 88 connects conduit I19 to chamber I8 of the valve I8. A branch conduit I8I, in which is interposed a check valve I 82, connects the vacuum line I11 to a vacuum reserve tank I83 on the trailer vehicle. Three branch conduits connect to the conduit I 8I, a branch conduit I84 connecting the conduit I8I to the forward end of the cylinder I16, a branch conduit I85 connecting the said conduit to port 29 of the control valve I8, and a third branch conduit I86 running from a point on conduit I8I between the manifold and the check valve I82 to chamber I1 of the valve I8. A pair of lines I81 and I88 connect an air cleaner I89 respec-' tively to chamber I9 of the valve I8 and to port 28 of the valve I8. A conduit I98 connects the rearward end of the power cylinder I16 to the port 38 of valve I8. The chambers of the valve I8 are thus connected in this use of applicants invention as follows: chamber I1 is connected to the vacuum line, chamber I8 is connected to the control line, and chamber I9 is connected to atmosphere.

Operation of this hookup is as follows. During brake release the tractor cylinder is atmospheric suspended and air at atmospheric pressure is therefore admitted to chamber I8 of the valve I8.

20 owing to the vacuum suspended condition of the trailer power cylinder when the brakes are released. The rod 2| is therefore at its upper-- ward, closing vacuum port 29 and opening air a port 28. Air is now admitted through ports 28 and 30 and conduit I90 to the rear of power cylinder I16, applying the trailer brakes.- It will be noted that the conduit 186 is connected to the conduit l8l forward of the check valve I82.

At the same time vacuum prevails in chamber hookup the connections will be the same as those described for the vacuum suspended tractoratmospherlc suspended trailer hookup. However, the rod 22l will be maintained in its uppermost position during brake release. Under this condition vacuum will be present in chambers 2H and 220, and atmosphere will be present in chambers 2l8 and 2l9. Actuation oi the tractor brakes will cause the admission of vacuum to chamber 2I8, which will in turn cause rod 22! to move downward closing vacuum port 229, opening air port 228 and admitting air to the trailer power cylinder to apply the trailer brakes. The valve will be balanced, i. e., in lapped position, when the downward pull of the vacuum in chamber 2l8 plus the downward pressure of the atmosphere in chamber 218 is equal to the upward This connection guarantees that the trailer brakes will be automatically applied in case of breakaway. Air will, if line I86 is broken, rush into chamber l1, causing the rod 2| to move downward and thus function to app y the trailer brakes;

Figure 6 shows a modification of the valve of Figure 1. The valve of Figure 6 is generally termed a. conversion valve because it functions to change a drop in pressure on the tractor brakes into a rise in pressure on the trailer brakes or vice versa. It is used chiefly with the combination of vacuum suspended tractor brakes and atmospheric suspended trailer brakes.

Since the valve of Figure 6 is in many respects similar to the valve of Figure 1 we will use the same numerals to designate the similar parts with the addition of 200. Thus the valve 2|! of Figure 6 is divided by dlaphragms 2H and 2 i 5 and a metal partition 2l6 into four chambers-2H, 2", 2|! and 220. Chamber 220 is provided with an air port 228, a vacuum port. 229 and a port 230 which is connected to the trailer power motor. The valve 210 is internally ported, having a passage 2I'la which-connects the chamber 2" with the vacuum port 229, and having a passage 2i9a which connects the chamber m with the air port 228. The chamber 2I8 is connected by a port 22' to the control line from the tractor power brakes.

Assuming that the valve is to be used with a vacuum suspended tractor-atmospheric suspended trailer hookup, during brake release rod 22! will be in its lowermost position, chamber 2i! will be maintained in vacuum IrOm the control line, and chamber 220 will be open to air at atmospheric pressure. Thus diaphragm 215 will be balanced between the vacuum in chamber 2H and that in chamber 2l8, while the diaphragm 2 will be balanced between the atmospheric pressure present in chamber 2l9 and the atmospheric pressure of chamber 220. Actuation of the tractor brakes will cause air to be admitted to chamber 2I8, 'unbalancing the forces over diaphragm 2l5 and moving rod 22l upward to first close the air port 228 and subsequently open the vacuum port 229 admitting vacuum to the trailer power cylinder to apply the trailer brakes. In case of breakaway, the control line will be broken and air will be admitted to chamber 2" causing application of the trailer brakes.

I! the valve m is used with an atmospheric suspended tractor-vacuum suspended trailer pull of the vacuum in chamber 211 plus the upward push of the atmospheric'pressure in chamber 220.

While we have described two specific embodiments of our invention, it is not our intention to be limited in scope to the particular embodiments described nor to be limited otherwise than by the terms of the appended claims.

We claim:

1. A valve for correlating the operation of vacuum brakes comprising a casing provided with four chambers, a flexible diaphragm placed between two 01 said chambers, a second flexible diaphragm placed between the other two of said chambers, means for maintaining one of said chambers under control line pressure, means for maintaining another of said chambers under controlled pressure, a floating control element connected to each of said diaphragms and movable therewith when the diaphragms flex owing to variations in pressure in the said chambers, a floatmg lever attached to the floating control element, and a pair of poppet type valve elements controlled by said floating lever.

2. A valve for correlating the operation oi vacuum brakes comprising a casing provided with four chambers, a flexible diaphragm placed between two of said chambers, a second flexible diaphragm placed between the other two oi said chambers, means for maintaining one of said chambers under control line pressure, means for maintaining another of said chambers under controlled pressure, a floating control element connected to each or said diaphragm and movable therewith when the diaphragms flex owing to variations in pressure in the said chambers, a pair of poppet type valve elements controlled by said floating control element, an atmospheric port controlled by one of said valve elements, and a vacuum port controlled by the other of said valve elements. I

3. A valve for correlating'the operation of vacuum brakes comprising a casing provided with four chambers, a flexible diaphragm placed between two of said chambers, a second flexible diaphragm placed between the other two 0! said chambers, means for maintaining one of said chambers under control line pressure, means for maintaining another of said chambers under controlledpressure, a floating control element connected to each of said diaphragms and movable therewith when the diaphragms flex owing to varations in pressure in the said chambers, a pair of poppet type valve elements controlled by said floating control element, an atmospheric port controlled by one of said valve elements, and a vacuum port controlled by the other of said valve elements, each oi said valve elements movin to Til?" including a vacuum chamber, an atmospheric her, a chamber connected to a control line, and a chber connected to a controlled motor, one of said passages connecting the vacuum chamber to a source of vacuum, a flexible diaphragm between the atmospheric chamber and the chamber connected to a controlled motor, and a second flexible diaphragm between the vacuum chamber and the chamber connected to a control line.

5. A valve for correlating the operation of vacuum motors comprising a casing having a plurality of passages and a plurality of chambers including a vacuum chamber, an atmospheric chamber, a chamber connected to a control line, and a chamber connected to a controlled motor, one of said passages connecting the vacuum chamber to a source of vacuum, another of said passages connecting the atmospheric chamber to atmosphere, a flexible diaphragm between the atmospheric chamber and the chamber connected to a controlled motor, and a second flexible diaphragm between the vacuum chamber and the chamber connected to a control line.

6. In combination with a truck and trailer each provided with a servo motor brake of the fluid pressure vacuum operated type, a source of vacuum, a main control valve on the truck, means including said valve for controlling the servo motor brake on the truck, a relay valve on the trailer connected to said main control valve and means including said relay valve for controlling the servo motor brake on the trailer, said relay valve compresing a casing having a pair of valve seats arranged on opposite sides of the easing, a pair of poppet valves for co-action with said seats, oppositely directed stems on the valve extending across the casing in parallel relation with each other, a floating lever connected to the stems, means for operating said lever for first closing one valve and then opening the other, and pressure responsive means for actuating said operating means in accordance with the relative values of four fluid pressures, of which one corresponds to the pressure of said main control valve, another to the pressure in the valve port connected to the trailer servo motor, another to I the pressure of the source of vacuum, and a fourth to atmospheric pressure.

7. In combination, a universal control valve for the trailer of a truck-trailer fluid pressure braking equipment, a source of fluid pressure, a brake on said trailer, a servo motor for operating said brake in accordance with adjustment of said valve, a main control valve on the truck to opcrate the brake by said servo motor, a control pressure line extending from said control valve on the truck to said valve on the trailer, said universal control valve having a plurality of ports, a plurality of poppet valve elements for controlling the passing of pressure fluid through said ports, and a movable element for opening and closing said ports in response to variations in pressure in said control line, in said servo motor and in said source of fluid pressure to one the servo motor to apply the brake upon either a decrease or an increase in the pressure in said control line depending upon the connection of the said valve in the system.

8. In combination with a truck and trailer each provided with a servo motor brake of the fluid pressure vacuum operated type, a source of vacuum. a main control valve on the truck, means including said valve for controlling the servo motor brake on the truck, a relay valve on the trailer connected to said main control valve, and means including said relay valve for controlling the servo motor brake on the trailer, said relay valve comprising a casing having a pair of valve seats, a pair of poppet valves for co-action with said seats, a floating lever connected to the valves, means for operating said lever for first closing one valve and then opening the other, and pressure responsive means for actuating said operating means in accordance with the relative values of four fluid pressures, of which one corresponds to the pressure of said main control valve, another to the pressure in the valve port connected to the trailer servo motor, another to the pressure of the source of vacuum, and a fourth to atmospheric pressure.

9. In combination with a truck and trailer each provided with a servo motor brake of the fluid pressure vacuum operated type, a source of vacuum, a main control valve on the truck, means including said valve for controlling the servo motor brake on the truck, a relay valve on the trailer connected to said main control valve, and means includingsaid relay valve for controlling the servo motor brake on the trailer, said relay valve comprising a casing having an atmospheric port and a vacuum port, a pair of valve members for controlling the atmospheric and vacuum ports, a floating lever connected to the valve members, means for controlling the floating lever to open and close said ports, and pressure responsive means for actuating said last-named means in accordance with the relative values of four fluid pressures, of which one corresponds to the pressure of said main control valve, another to the pressure in the valve port connected to the trailer servo motor, another to the pressure of the source of vacuum, and a fourth to atmospheric pressure.

10. In combination, a universal control valve for, the trailer of a truck-trailer fluid pressure braking equipment, a source of fluid pressure, a brake on said trailer, a servo motor for operating said brake in accordance with adjustment of said valve, a main control valve on the truck to operate the brake by said servo motor, a control pressure line extending from said control valve on the truck to said valve on the trailer, said universal control valve having four pressure chambers each of which is disconnected from all the. other chambers and having a plurality of ports and a movable element for opening and closing said ports in response to variations in pressure in said control line, in said servo motor and in said source of fluid pressure to energize the servo motor to apply the brake upon either a decrease or an increase in the pressure in said control line depending upon the connection of the said valve in the system.

HENRY BAADE. THOMAS H. .1 1. 

